I think that you can get a couple patches and it will spread. I bet if you buy more though the bottom of the tank will be filled more quickly. Mine died sadly so I don't know much about it.. but Im almost positive it spreads!

here's what my java moss "carpet" looked like
it was tied to a clay rectangle plate. it filled in after about 4-6 months and looked decent. it went out of control after a year or two, which is why i don't have it anymore

try to find a fuller kind of moss. xmas moss or taiwan or pheonix moss will grow more dense in my opinion